1. Field of the Invention
This invention relates to a fiber optic transducer which uses a fluid to couple a sensor to a fiber optic medium.
2. Description of Related Art
Optical fiber sensor transducers are commonly used in the measurement of magnetic fields H and electric fields E. In response to either a magnetic or electric field, a magnetostrictive or electrostrictive sensor material, respectively, changes its physical geometry. When the direction of interest is the sensor length, L, the proportional change in sensor length is called the strain: EQU e=.DELTA.L/L (1)
where e is the strain, L is the starting length of the sensor and .DELTA.L is the change in length. A coupling material, such as an adhesive, binds the sensor material and optical fiber together and transfers the strain from the sensor material to the optical fiber, causing the length of the fiber to correspondingly change. As light travels through the optical fiber, the change in the length of the fiber changes the path length of the light, creating a phase shift in the light at a receiving end of the optical fiber.
A known fiber optic transducer uses materials in which the strain can be expressed as a nonlinear function of H or E. Typically EQU e=CH.sup.2 ( 2)
for magnetostrictive materials, and EQU e=ME.sup.2 +QE (3)
for electrostrictive materials, where the coefficients C, M, and Q depend on the transducer sensor material, the transducer geometry, and the frequency. C is the magnetostrictive coefficient, M is the electrostrictive coefficient and Q is the piezoelectric coefficient. These coefficients are determined by the sensor material geometry and the frequency of the measured field.
The frequency of the measured electric or magnetic field is the rate at which the field's intensity or amplitude is changing with respect to time. The amplitude of a DC field is constant in time while the amplitude of an AC field varies with time. The sinusoidal time variation in the amplitude of the AC field is given by hsin(.omega.t), where h is the peak amplitude, .omega. is the frequency of the sine wave in radians per second, and t is time in seconds. When the magnetostrictive or electrostrictive sensor is exposed to a sinusoidal magnetic or electric field, the strain e varies sinusoidally at the same frequency. When the sensor is simultaneously exposed to two sinusoidal magnetic or electric fields having frequencies of .omega. and .OMEGA. and amplitudes h and H respectively, the strain is derived by standard techniques and expressed as the sum and difference of the frequencies .omega. and .OMEGA.: EQU e(.omega..+-..OMEGA.)=ChH.sub..OMEGA. sin(.omega..+-..OMEGA.)t(4)
Usually the higher of the two frequencies is called the carrier frequency, and the lower of the two frequencies is called the signal frequency.
These fiber optic transducers are conventionally used to measure a low frequency magnetic field H.sub..OMEGA. sin(.OMEGA.t) by applying a higher frequency field hsin(.omega.t) into the magnetostrictive material and detecting the strain at the sum and the difference frequencies (.omega.+.OMEGA. and .omega.-.OMEGA., respectively). Since h and .omega. are known, H.sub..OMEGA. and .OMEGA. are easily obtained from equation (4). By coupling the magnetostrictive material to an optical fiber containing traveling light waves, the strain transferred from the magnetostrictive sensor material to the optical fiber induces a shift in the phase of the light wave by EQU .phi.(.omega..+-..OMEGA.)=2.pi.n.xi.L.eta.ChH.sub..OMEGA. sin(.omega..+-..OMEGA.)t (5)
where .phi. is the phase shift, n is the index of refraction of the fiber core of the optical fiber, .xi. is the strain-optic coefficient, L is the length of the optical fiber which is attached to the magnetostrictive sensor material, and .eta. is the strain-transfer coupling efficiency. The phase shift is easily detected by standard interferometer techniques.
When the sensor material is coupled to the optic fiber using hard adhesives, such as epoxy, the adhesive itself introduces dimensional constraints on the transducer. Since the change in physical length is the only mechanism available for the sensor material to transfer the detected magnetic or electric field to the fiber optic cable, the hard adhesive is not an ideal coupling material because it interferes with this mechanism by restraining the sensor material, preventing it from freely expanding or contracting in response to the applied or measured field. In addition, the adhesive responds to environmental conditions, such as temperature, humidity and other time dependent parameters, independently of the sensor.
Accordingly, hard adhesives cause three deleterious effects:
1) the adhesive restricts the strain of the sensor material. This mechanical loading reduces the effective strain generated per applied field; 2) time-dependent variations in the adhesive cause time-dependent fluctuations in the coupling factor .eta.; 3) for magnetostrictive sensors, the adhesive induces local strains in the magnetostrictive sensor material, thus reorienting the local magnetic easy axis and decreasing the magnetostrictive response C. Additionally, all three effects interfere with the ability to reproduce substantially similar responses from transducer to transducer.